builtins: accept old-style iterables to iter (#7817)

test_cases/stdlib/builtins/test_iteration.py

@@ -0,0 +1,14 @@
+from typing import Iterator
+from typing_extensions import assert_type
+
+
+class OldStyleIter:
+    def __getitem__(self, index: int) -> str:
+        return str(index)
+
+
+for x in iter(OldStyleIter()):
+    assert_type(x, str)
+
+assert_type(iter(OldStyleIter()), Iterator[str])
+assert_type(next(iter(OldStyleIter())), str)